1. Field of the Invention
The invention relates to event time period and heart rate measurement, and more particularly, to a user controllable method to measure times between displayed events and to measure heart rate. 2. Description of the Prior Art
Cardiac electrophysiology is a subspeciality within the field of cardiology which attempts to study heart rhythm disorders. Heart irregularities can range from occasional extra beats to life threatening fast or slow rhythms. Treatments for these problems include reassurance, drugs, pacemakers, cardiac surgery, electrical ablation of nerve conducting paths in the heart, and implantable automatic defibrillators which can recognize malignant rhythm disorders and produce an electric shock to the heart to terminate the rhythm.
Though most heart rhythm disorders can be treated by non-electrophysiologists, failure of the patient to respond to conventional therapy or presentation of a patient with a malignant rhythm requiring intensive intervention results in referral of the patient to the electrophysiologist. In order to understand the nature of the disorder, the physician will perform an electrophysiological study on the patient. The purpose of the study is to measure the conduction parameters of the heart and to reproduce the rhythm disorder if possible.
The study starts by placing the patient recumbent in a well equipped cardiac laboratory, sedating the patient and placing catheters into various parts of the heart. The catheters are placed by accessing large veins and arteries in the body, puncturing these vessels and then threading the thin catheters, typically having diameters of 2 to 4 mm, into the vessels, guiding the catheters toward the heart while monitoring the catheter position by X-ray, and placing the catheters appropriately within the specific chambers of the heart to be analyzed. The catheters have a number of electrode locations spaced along the end of the catheter to allow various types and locations of electrical measurements to be performed. The electrode leads from the catheter are connected to amplifiers to increase the signal level from the 2-50 mV range to more useable levels. These amplified signals are typically supplied to an oscilloscope for direct observation and to a storage device. The storage devices are typically a magnetic tape unit or a light sensitive paper recording unit. The signals received from the electrodes represent the activation of the heart muscle or the conduction by nerve tissue which carries the activation signal to various parts of the heart in order to excite and thus cause contraction of the heart muscle, resulting in the pumping of blood.
Two to four catheters are placed in a typical study and each catheter may have several electrodes on it, providing typically 2-12 signals to view. Additionally, there are usually 3 or 4 body surface leads attached and simultaneously monitored.
In order to study the conduction controllable characteristics of the heart, one of the catheters is connected to a pacemaker unit which provides pulses of electricity similar to the hearts natural pacemaker. In one type of test, a series of uniformly spaced impulses is provided to cause the heart to pump. An odd impulse is provided a selectable time after a uniformly spaced impulse. If the impulse does not cause a beat, the odd impulse is within the refractory period. A group of these series are performed and the desired drug introduced into the patient. The test protocol is repeated to determine if any changes have occurred because of the drug.
A second type of test uses different impulse waveforms and trigger locations to attempt to get the heart into the malignant rhythm. If this can be done, the various locations, signal types, and timing intervals provide information to identify the problem so that the desired treatment can be performed.
In yet a third test, the heart is paced until a uniform rhythm is obtained, then the pacing signal is stopped. The heart's own pacemaker should take over and begin controlling the pumping. The time interval and various conduction paths identify certain problems which may be treated.
All of the information is recorded during the various tests or protocols. The entire session or study may last from 15 minutes to many hours, with individual test series lasting seconds to minutes. The data which has been recorded must then be analyzed. When the laboratory uses light sensitive paper as the recording medium, as most laboratories do, the physician must use mechanical hand calipers to take the time measurements necessary for proceeding with analysis. The physician must sort through many feet of paper with the hand calipers. This technique is quite cumbersome and takes a significant amount of time if done properly. This large use of time increases the cost and complexity of the study and thus the cost to the patient.
The patient's heart rate must also be monitored during the study to help keep stress levels low. Previous methods of determining the heart rate have been relatively slow and inaccurate, limiting the ability to carefully monitor the patient's condition.
Additionally, at times during the study the patient may enter ventricular tachycardia, a life threatening condition where the heart stops pumping blood correctly. This condition should be stopped as quickly as possible. This is generally done by changing the pacing signal timing to approximately 70% of the ventricular tachycardia rate and then controlling the pacing once the system has regained control of the heart. This ventricular tachycardia rate has been difficult to measure previously because the signal appears only on an oscilloscope or as a trace on the paper. The interval must be measured and converted to a form to allow setting the pacing signal rate. This process should occur as rapidly as possible to prevent possible complications, but the current cumbersome technique limits the speed.
Digital oscilloscopes have had the ability to indicate the time between two pointers, but conventionally the pointers have been small symbols which are hard to recognize and have been hard to manipulate